This invention relates to starting gas turbine engines, and, more particularly, to facilitating gas turbofan engine restarts by effectively altering the nozzle exit area.
Gas turbine engines are widely known and used for power generation and vehicle (e.g., aircraft) propulsion. During in-flight propulsion of a multi-engine aircraft, certain problems may occur with one engine causing the engine to shut down. For example, inclement weather, non-optimum trimming of engine idle, fuel nozzle coking, fuel contamination, loss of electric power, fuel mismanagement, pilot error, or the like may, under certain conditions, warrant voluntary or automatic shut down of an engine. Although the remaining engines can typically fly the aircraft, it is ordinarily desired to restart the shut down engine while the aircraft is still in-flight.
An engine restart envelope includes combinations of aircraft altitude and airspeed that provide a suitable air supply to the engine sufficient for restarting. When traveling outside of the engine restart envelope, the air supply to the engine may not contain enough oxygen to support combustion during ignition. In some instances, starter-assistance may be used to increase the rotational speed of a fan section of the engine, which increases altitude and airspeed combinations suitable for restarting the engine. Increasing the rotational speed of the fan section draws additional airflow into the engine and, in so doing, augments the supply of oxygenated air supporting combustion.
Disadvantageously, at certain combinations of altitude and airspeed, increasing the rotational speed of the fan section is not alone sufficient to generate adequate airflow to support combustion. As a result, aircraft experiencing in-flight shutdown may have to rapidly adjust altitude and/or airspeed to move within the engine restart envelope or starter-assisted engine restart envelope. As an example, if an engine requires restart in aircraft traveling at an altitude unsuitable for engine restarts, the aircraft may rapidly decrease elevation to move to an altitude suitable for restarting the turbofan engine. Alternatively, the aircraft may be forced to continue flying, without propulsion from the shutdown engine.
What is needed is a method capable of restarting the turbofan engine through an increased number of altitudes and airspeeds.